(1) CONVERSION OF ALCOHOL INTO ALKYL CHLORIDE: Alkyl chlorides can be prepared from corresponding alcohols by the reaction with PCl₅ (yellow white solid) at room temperature. PBr₅ converts alcohols to alkyl bromide. PI₅ is highly unstable. The reaction take place by SN₂ mechanism (inversion in configuration) where PCl₅ gets converted into POCl₃ and HCl. Phosphorous Trihalides (PX₃) can also be used for the conversion of alcohol into alkyl halide in the same manner (PX₃ gets converted into H₃PO₃), PCl₃ and PBr₃ are stable but PI₃ is unstable so it is generated in situ by heating red P with I₂. Please note generally primary and secondary alcohols are converted into alkyl halide, tertiary alcohols mainly undergo elimination so alkyl halides are not obtained in a good yield.

(2) CONVERSION OF CARBOXYLIC ACID INTO ACID CHLORIDE AND PRIMARY AMIDES TO NITRILES: Acid chlorides can be obtained by reacting carboxylic acid with PCl₅ or PCl₃. Acetic anhydride gives acetyl chloride. Primary acid amides (RCONH₂) are converted into RCN by dehydration.

(3) ALDEHYDES AND KETONES are converted into terminal and non-terminal geminal dichlorides respectively.

(4) BECKMANN REARRANGEMENT (CONVERSION OF KETOXIMES INTO AMIDES): Ketoximes (RR\’C=NOH) when treated with acidic reagents such as PCl₅, they rearrange to substituted amides (RCONHR’ or R’CONHR). The group that migrates is generally the one anti to -OH of oxime but when attached alkyl groups of oxime are both bulky then mixtures of two amides are possible (RCONHR’ and R’CONHR). The oximes of cyclic ketones give ring enlargement. Among other reagents used are SOCl₂, Concentrated H₂SO₄, HCOOH, Liquid SO₂, HMPA, P₂O₅-Methanesulphonic acid, HCl-HOAc-AC₂O, Silica gel and polyphosphoric acid.